(1) Field of the Invention
The present invention relates to an optical modulator suitable for use as an external optical modulator for modulating light emitted from a signal light source, in a transmitting unit of, for example, an ultra high-speed optical communication system.
(2) Description of Related Art
There has been used, in a transmitting unit of an optical communication system, an optical modulator in a direct modulation system which modulates an electric current flowing in a semiconductor laser with a data signal, as an optical modulator which modulates light emitted from, for example, a semiconductor laser as a signal light source.
However, a recent demand for high-speed optical communication system is accompanied with a necessity for high-speed optical modulation. If light is modulated at a high speed using such optical modulator in a direct modulation system, an effect of a wavelength fluctuation (chirping) of emitted signal light becomes greater, which leads to wavelength dispersion in an optical fiber. As a result, a long-distance transmission becomes difficult.
It is therefore necessary to use an external modulator which does not generate chirping in principle when light is modulated at a high speed. As an example of the above external modulator, there is an optical modulator of a Mach-Zehnder type shown in FIG. 29. Incidentally, FIG. 30 is a perspective view of the optical modulator 100 of a Mach-Zehnder type shown in FIG. 29, looking from an outputting side thereof.
As shown in FIG. 29, the optical modulator 100 of a Mach-Zehnder type has an optical waveguide device 101 of a Mach-Zehnder type, a photo-detector 107 and a signal controlling circuit (ABC circuit; automatic bias control circuit) 108.
An optical waveguide 104 of a Mach-Zehnder type is formed on a substrate 101a, further a travelling-wave electrode 102 and a grounding electrode 103 are formed on the optical waveguide 104, whereby the optical waveguide device 101 of a Mach-Zehnder type is formed.
The substrate 101a is a lithium niobate (LiNbO.sub.3) substrate having electrooptic effect.
The optical waveguide 104 of a Mach-Zehnder type has an input waveguide 104a, an output waveguide 104b and intermediate waveguides 104c and 104d. The intermediate waveguides 104c and 104d are disposed in parallel, and connected to the input waveguide 104a and the output waveguide 104b at a Y-shaped splitting portion Q.sub.1 and a Y-shaped recombining portion Q.sub.2.
A metal such as titanium (Ti) or the like is selectively diffused in only the optical waveguide portion in the substrate 101a of the optical waveguide 104, whereby a refractive index of that portion is larger than a refractive index of another portion.
The travelling-wave electrode 102 and the grounding electrode 103 are used to control light propagated in the optical waveguide 104, which are formed by evaporating a metal such as gold (Au) or the like on the intermediate waveguides 104c and 104d of the optical waveguide 104, as shown in FIG. 29.
Into the input waveguide 104a of the optical waveguide 104, direct-current light emitted from a semiconductor laser 111 is inputted through an optical fiber 105a. From the output waveguide 104b of the optical waveguide 104, modulated signal light is outputted to the photo-detector 112 through an optical fiber 105b.
The optical fiber 105a is a constant polarization fiber, whereas the optical fiber 105b is a single-mode fiber.
The photo-detector 112 receives the signal light outputted through the optical fiber 105b to convert it into an electric signal.
An optical fiber 106 is directly attached, in addition to the above-mentioned optical fiber 105b, onto an end of the substrate 101a on the outputting side, through which radiation light generated at the Y-shaped recombining portion Q.sub.2 of the optical waveguide 104 is inputted to the photo-detector 107.
Incidentally, the optical fiber 106 is a multi-mode fiber.
The photo-detector 107 receives the radiation light inputted through the optical fiber 106 to convert the received radiation light into an electric signal, and outputs the electric signal to the signal controlling circuit 108, thereby monitoring the radiation light.
The photo-detector 107 is connected to the signal controlling circuit 108, which varies direct-current bias of an inputted electric signal to be applied to the travelling-wave electrode 102 according to a result of the monitoring by the photo-detector 107 (that is, according to a change of a light output electric signal from the photo-detector 107).
In the optical modulator 100 of a Mach-Zehnder type, an operating point of the optical modulator 100 of a Mach-Zehnder type is generally shifted with time elapsed due to temperature drift, DC drift, stress and the like.
Now, shift of the operating point of the optical modulator 100 of a Mach-Zehnder type will be explained with reference to FIG. 31.
FIG. 31 is a diagram showing an input-output characteristic of the optical modulator 100 of a Mach-Zehnder type. In FIG. 31, 1 indicates the characteristic before the operating point is shifted, and 2 indicates the characteristic in the case where the operating point has been shifted.
As shown in FIG. 31, the input-output characteristic of the optical modulator 100 of a Mach-Zehnder type has periodicity to a driving voltage.
Use of driving voltages V.sub.0 and V.sub.1 at which an upper peak value and a lower peak value of an output light power are obtained according to a logical multiplication of an input signal enables efficient binary modulation.
However, if the driving voltages V.sub.0 and V1 are constant even in the case where the operating point is shifted, an extinction ratio of a signal light outputted from the optical modulator 100 of a Mach-Zehnder type is degraded because of the above-mentioned periodicity as shown in FIG. 31.
When the operating point is shifted, it is therefore necessary to control the operating point assuming the driving voltages V.sub.0 and V.sub.1 as (V.sub.0 +dV) and (V.sub.1 +dV), respectively, if a quantity of the shift is dV.
In the optical modulator 100 of a Mach-Zehnder type shown in FIG. 29, the photo-detector 107 monitors radiation light generated at the Y-shaped recombining portion Q.sub.2 of the optical waveguide 104, the signal controlling circuit 108 varies direct-current bias of an input electric signal to be applied to the travelling-wave electrode 102, whereby the operating point of the optical modulator 100 of a Mach-Zehnder type is controlled.
Incidentally, reference numeral 109 denotes an input signal source, and reference numeral 110 denotes a termination resistor.
In the optical modulator 100 of a Mach-Zehnder type with the above structure shown in FIG. 29, direct-current light (incident light) from the semiconductor laser 111 is inputted to the input waveguide 104a of the optical waveguide 104 through the optical fiber 105a, split into two at the Y-shaped splitting portion Q.sub.1, then propagated in the intermediate waveguides 104c and 104d.
If a high-frequency modulating signal voltage is applied to the travelling wave electrode 102 at this time, a phase difference is generated between the split incident lights by the electrooptic effect in the intermediate waveguides 104c and 104d, and the incident lights whose phases are different are again combined at the Y-shaped recombining portion Q.sub.2.
By setting the driving voltages at this time such that the phase difference between the incident lights in the intermediate waveguides 104c and 104d is 0 and .pi., an ON/OFF optical pulse signal can be obtained as signal light to be outputted, and modulated signal light is outputted from the output waveguide 104b of the optical waveguide 104.
The signal light outputted from the output waveguide 104b is received by the photo-detector 112 through the optical fiber 105b to be converted into an electric signal.
On the other hand, the radiation light generated at the Y-shaped recombining portion Q.sub.2 of the optical waveguide 104 is received by the photo-detector 107 through the optical fiber 106, converted into an electric signal, then outputted to the signal controlling circuit 108.
The signal controlling unit 108 varies direct-current bias of the input electric signal to be applied to the travelling-wave electrode 102 according to a change of the light output electric signal from the photo-detector 107, thereby controlling the operating point of the optical modulator 100 of a Mach-Zehnder type.
According to the optical modulator 100 of a Mach-Zehnder type shown in FIG. 29, the photo-detector 107 monitors radiation light generated at the Y-shaped recombining portion Q.sub.2 of the optical waveguide 104, the signal controlling circuit 108 controls the operating point of the optical modulator 100 of a Mach-Zehnder type on the basis of a result of the monitoring by the photo-detector 107, whereby the operating point of the optical modulator 100 of a Mach-Zehnder type can be stabilized. As a result, it is possible to prevent degradation of signal light as shown in FIG. 31 so that stable optical modulation is possible.
The optical modulator 100 of a Mach-Zehnder type shown in FIG. 29 has, however, disadvantages that it is necessary to align the optical fiber 106 since the optical fiber 106 is used when the radiation light generated at the Y-shaped recombining portion Q.sub.2 of the light waveguide 104 is monitored, and that there is a difficulty in designing the structure.
Namely, the optical fiber 105b to which modulated signal light is inputted and the optical fiber 106 to which radiation light generated at the Y-shaped recombining portion Q.sub.2 of the light waveguide 104 is inputted are spaced only about 80 .mu.m apart so that fabrication of the optical modulator 100 of a Mach-Zehnder type is difficult.